Abstract

Background

All jawed-vertebrates have four T cell receptor (TCR) chains: alpha (TRA), beta (TRB),
gamma (TRG) and delta (TRD). Marsupials appear unique by having an additional TCR:
mu (TRM). The evolutionary origin of TRM and its relationship to other TCR remain
obscure, and is confounded by previous results that support TRM being a hybrid between
a TCR and immunoglobulin locus. The availability of the first marsupial genome sequence
allows investigation of these evolutionary relationships.

Results

The organization of the conventional TCR loci, encoding the TRA, TRB, TRG and TRD
chains, in the opossum Monodelphis domestica are highly conserved with and of similar complexity to that of eutherians (placental
mammals). There is a high degree of conserved synteny in the genomic regions encoding
the conventional TCR across mammals and birds. In contrast the chromosomal region
containing TRM is not well conserved across mammals. None of the conventional TCR
loci contain variable region gene segments with homology to those found in TRM; rather
TRM variable genes are most similar to that of immunoglobulin heavy chain genes.

Conclusion

Complete genomic analyses of the opossum TCR loci continue to support an origin of
TRM as a hybrid between a TCR and immunoglobulin locus. None of the conventional TCR
loci contain evidence that such a recombination event occurred, rather they demonstrate
a high degree of stability across distantly related mammals. TRM, therefore, appears
to be derived from receptor genes no longer extant in placental mammals. These analyses
provide the first genomic scale structural detail of marsupial TCR genes, a lineage
of mammals used as models of early development and human disease.